SECT. 3] AND ORGANISATION 583 



demonstrate animal form from appliances of convenience into de- 

 scriptions of actual fact, and to have substituted for morphological 

 axes, axes of physico-chemical difference. Difficulty was bound to 

 arise when some sort of identification of the physico-chemical dif- 

 ferences was attempted, and Child, content with a loose and general 

 association, chose metabolic rate as the physico-chemical variable. 

 "Axial gradients have often been called metabolic gradients", he 

 said, "because differences in metabolism, or, more specifically, of 

 oxidative metabolism, as indicated by various experimental methods, 

 appear to be characteristic and conspicuous features of them." It 

 will be necessary presently to examine the evidence on which this 

 statement is based, but first of all a few theoretical remarks require 

 attention. 



That gradients of various kinds exist within the developing embryo 

 has long been known. The "law of developmental direction" 

 (Jackson and Scammon) or of "cephalocaudal differential growth" 

 (Calkins), which we have already discussed in relation to the growth 

 of parts in the embryonic body, is simply, after all, a statement of 

 the fact known in general to Aristotle, that the head end of an 

 embryo develops quicker than the tail end. Again, as Minot showed, 

 the cephalic somites develop before the more caudal ones. "The first 

 parts to become morphologically visible", as Child puts it, "are the 

 apical or anterior regions, and these are followed in sequence by 

 the successively more posterior or basal parts." Again, there are 

 dorso- ventral gradients. In those bilaterally symmetrical inverte- 

 brates which have a ventral nerve-cord (including most worms and 

 arthropods) the ventral and median regions of the embryo at any 

 given level of the body develop more or less in advance of the dorsal 

 and lateral regions. On the other hand, in vertebrates, where the 

 nerve-cord occupies a dorsal position, the dorso-ventral gradient runs 

 the opposite way, and differentiation and growth proceed more 

 rapidly in the median dorsal region than in the lateral and ventral 

 regions. The antero-posterior gradient, however, is the same as in 

 the invertebrates. For a recent discussion of the law of cephalocaudal 

 differential growth Kingsbury's papers should be consulted. 



Again, the rule which has been named after F. M. Balfour, that 

 the rate of cleavage in an embryonic region is inversely proportional 

 to the amount of yolk which the cells in it contain, is associated with 

 the gradient system of the egg. For the apicobasal gradient in the 



